Surface plasmon resonance is being used in biosensing, in such areas as immunoassay and nucleic acid detection Basically, surface plasmons are electromagnetic waves created along an interface between a conducting material and a non-conducting material. A common technique for their creation is to direct a beam of electromagnetic radiation into a glass prism with an angle of incidence above the critical angle so that it undergoes total internal reflection. The internal reflection creates an evanescent electromagnetic wave at a region outside of the prism adjacent to the surface. When a thin conductive film is deposited on the surface of the prism, surface plasmons will be formed.
Surface plasmon resonance occurs when the momentum (or the wave vector) and energy (i.e. frequency) of the evanescent electromagnetic wave are made to match the momentum and energy of the surface plasmons respectively. It is characterized by a sharp decrease in intensity of the reflected beam as its energy is transferred, because of the resonance, to the surface plasmons.
The wave vector K.sub.e of the evanescent wave is defined by the equation: EQU K.sub.e = (.omega./C) n sin.theta.,
where .omega. is the angular frequency of the incident beam, c is the speed of light in vacuum, n is the refractive index of glass and .theta. is the angle of incidence- The wave vector of the surface plasmon is defined by the equation: EQU K.sub.sp = (.omega./c) (1/.epsilon..sub.m + 1/.epsilon..sub.s).sup.-1/2,
where .epsilon..sub.m is the real part of the dielectric constant of the metal and .epsilon..sub.s is the dielectric constant of the substance under test (or in the absence of any substance, of air) surrounding the metal.
At resonance, the wave vector of the evanescent wave is the same as that of the surface plasmons so that there is no electromagnetic wave reflected from the surface. Therefore, occurrence of the surface plasmon resonance is given by the equation: EQU K.sub.e = K.sub.sp.
If a periodic structure such as a grating or a surface acoustic wave is impressed upon the thin metal layer, the above equation becomes: EQU K.sub.e + k = K.sub.sp.
where k is the wave vector due to the periodic structure.
The above equation provides a useful tool for measuring differences between the values of .epsilon..sub.s of different materials. It also provides a useful tool for detecting the presence of trace surface chemicals in a substance that alters its .epsilon..sub.s value. By measuring the differences of K.sub.e at resonance, the changes in .epsilon..sub.S can be determined.
Surface plasmon resonance measuring instruments heretofore known which utilize the above equality condition have all measured the differences of K.sub.e by varying .theta. and sensing the reflected beam at different values of .theta., as generally shown in FIG. 1 to detect the resonance. In these prior art surface plasmon resonance measuring instruments, sensing the reflected beam at different values of .theta. has been accomplished by three known methods.
Under a first method, the position of the source of electromagnetic radiation is fixed. The prism is rotated in order to change the value of .theta.. The detector for detecting the reflected beam is also rotated by 2.theta..
Under a second method, as disclosed in "The ATR Method With Focused Light -- Application to Guided Waves On A Grating" by E. Kretschmann, Vol. 26, number 1, Optics Communications, 1978, and in U.S. Pat. No. 4,997,278, entitled "Biological Sensors", issued Mar. 5, 1991 to Finlan et al, both the source and the prism are fixed. Refractive optics are then employed to spread the incident light into a cone of light beams with different values of .theta.. The reflected beams are then detected by an array of diodes.
Under a third method, disclosed in European Patent Application Number 89304570.8, filed on May 5, 1989 by Finlan et al, the positions of both the light source and the prism are fixed. A rotating mirror is used to direct the source light to the prism at different angles of incidence. The reflection of the source beam is then deflected by another curve mirror into the detector.
All the above-described methods have relied upon moving mechanical parts to generate and/or detect the surface plasmon resonance. The disadvantages of a reliance upon moving mechanical parts are obvious -- they are susceptible to fatigues, wear and tear, and they require precision tolerances that are difficult to create and maintain.
The second method suffers an additional disadvantage. Because the incident beam is spread, its intensity is reduced. In typical applications, however, the beam is additionally used to produce intensity dependent phenomenon such as fluorescence, and the effectiveness of the instrument would thus be adversely affected by the reduction of intensity resulting from this method.
The third method also suffers from other additional disadvantages. Because the beam may enter the detector from different angles, the detector needs to detect the reflected beam from different directions. Therefore, a detector having a large collection solid angle is required, and detectors having such capability are typically either expensive or less sensitive. The detector is also required to be placed at the focal point of the curved mirror, which is inconvenient and renders the overall system more susceptable to alignment errors.
Although there are other devices in the prior art that utilize surface plasmon resonance, they are not suitable for performing measurements.
What is needed is an instrument which can be used for measuring chemical compositions by measuring the dielectric constants thereof utilizing surface plasmon resonance, and one that does not have any moving mechanical part.